Variable coupling tunable microwave resonator



Oct 3, 1950 UNDER 2,524,532

VARIABLE COUPLING TUNABLE MICROWAVE RESONATOR Filed Feb. 27, 1946 2 Sheets-Sheet 1 INVENTOR.

Erma/2 G. [I'm/01' ATZUR/Vi) Filed Feb. 27, 1946 2 Sheets-Sheet 2 Oct. 3, 1950 E. G. 'LINDER 2,524,532

VARIABLE COUPLING TUNABLE MICROWAVE RESONATOR BYQQM- Patented Oct.

VARIABLE jCO'UPLIN TUNABLE MICRO WAVE RESONATOR Ernest G. Lin-der, Princeton, N. 'J.-, assignor to;

Radio Corporation of America, acorporation of Delaware Application February 27, 1946, Serial No. 650,710

com s. (c1. 17 8--44) This invention relates generally to microwave apparatus and. more ;particularly to improved variable-coupling wide=frequencyrband tunable microwave resonators ofzltheltype utilizing.a-,mov-

able piston wherein adjustment ofthe piston for tuning the resonator simultaneously adjusts the coupling characteristics of a coupling element projecting into the resonator.

It has been customary to utilize adjustably tuned microwave resonators as frequency controlling or filter elements in microwave generators, mixers and transmission systems. The resonant frequency of such microwave resonators may be varied by longitudinal adjustment of a conductive piston enclosed within the resonator shell. Well known methods of coupling into or out of such adjustable resonators include the use of microwave probe elements, coupling loops and adjustable aperture devices, depending upon the characteristics of the circuit to which the resonator is coupled. It is frequently advantageous'to be able to vary the degree of coupling between" an external circuit and the microwave resonator as the resonator is tuned throughits useful frequency band. In particular, in reflex oscillators of the type including an electron reflecting electrode, it is desirable that the coupling coefficient between the oscillator andthe'load circuit'remain substantially constant or decrease astheos cillator frequency is increased. A conventional coupling loop, probe or'aperture device coupled into the oscillator cavity normally will provide an increased coupling coefficient as the frequency is increased, thereby providing non-linear loading of the oscillator over its useful frequency range.

The instant invention comprises several embodiments of microwave apparatus. wherein the frequency of a microwave. resonator is varied by longitudinal adjustment of .a piston operative therein, and wherein the effective length of a microwave coupling, probe, or the. effective coupling of a coupling loop, as well as the effective opening of an aperture device islsimultaneously adjusted by movementof the tuning piston. The

invention as disclosed provides means for tuning a microwave generator and simultaneously adjusting the coupling thereto froman external coaxial line or waveguide device.

Among the objectslof theinvention are to provide an improved method of and. means for. simultaneously adjusting the tuningand coupling of a microwave resonator. device. Another obj ect of the invention is to providea methodof and relativewide frequency rangeand for simultaneously adjusting the couplingcoefiicient of .a. coupling element connected to an external circuit. A further'object of the invention is to provide an improved cylindrical microwave resonator including a longitudinally adjustable tuning piston, wherein adjustmentof the piston simultaneously adjusts the effective coupling to the resonatorof amicrowave coupling probe A still furtherobject of the invention is to provide an improved cylindrical microwave resonator including a 1ongitudinally adjustable tuning piston, wherein ad justment of the piston simultaneously adjusts the effective coupling to the resonator of a microwave coupling. loop. Another object is to provide an improved cylindrical microwave resonator including a longitudinally adjustable tuning pis-. ton, wherein adjustment of the piston simultaneously adjusts theeffective opening-of an aperture device for couplingan exte'rnalcircuitto said resonator.

An additional object is toprovide an improved variable-coupling wide-frequency-band micro.- wave resonator including a frequency adjusting pistonwherein adjustment of the tuning piston simultaneouslyvaries inv opposite sense the resonantfrequencylof aid=.resonator andthe coupling coefiicient of an element coupling said resonator toan external circuit; A furtherobject is to pro-v vide a constant-coupling wide-frequency-band microwave resonator including. a movable tuning piston whereinadiustment.of the tuning piston varies the resonant frequency of said resonator without substantialvariation of the coupling. coeflicient of an element coupling said, resonator to an external circuit. Another object is to provide an improved arrangement for simultaneously adjusting the tuning and coupling of a microwave resonator without the necessity of sliding contacts in the coupling element. 7

The invention will be described in greater detail by reference to the. accompanying drawing, of which Figure l is .a cross-sectional elevational view of a first embodiment of the invention; Figure 2 is a cross-sectional elevational view of a secondembodiment of the-invention; Figure 3 is a plan, partially cross-sectional, view along the section. line IIL-IIIof the device shown in Fig. 2; Figure 4 is a cross-sectional elevational view of a third embodimentof the invention; Figure 5 is a cross-sectional elevational. view of a fourth embodiment of the invention; Figure 6 is a crosssectional elevational view of a fifth embodiment of the invention; Figurefl is a schematic diagram means for tuning a microwave resonator .over a m of a modification of the device shown in Fig. 4;

Figure 8 is a schematic diagram of a modification of the device shown in Fig. Figure 9 is a schematic diagram of a first modification of the device shown in Fig. 6; and Figure is a crosssectional view of a second modification of the device shown in Fig. 6. Similar reference charaoters are applied to similar elements throughout the drawing.

Referring to Figure 1 of the drawing, a microwave resonator l includes a cylindrical shell 3 having a longitudinally adjustable piston 5 disposed therein. The piston position may be adjusted by means of a central shaft 1 actuated by any conventional actuating device, not shown. A coupling coaxial line 9 has its outer conductor i l fitted into an aperture I3 in the piston The inner conductor l5 of the coaxial line 9 is insulated from the outer conductor ii and is longitudinally movable with said outer conductor and the piston as the piston position is varied. The inner coaxial line conductor l5 extends into the cavity formed between the resonator Wall 3 and the inner face of the piston 5, and as the piston and coaxial line are advanced toward the end resonator wall it, the exposed end it? of the inner coaxial line conductor l5 telescopes within an outwardly projecting portion 2| of the resonator wall 3.

Thus, as the resonant frequency of the resonator I is increased by advancement of the piston face toward the end resonator wail H, increasingly shorter portions of the inner conductor l5 are exposed to the resonator cavity, thereby correspondingly tending to reduce the coupling between the cavity resonator and the coupling coaxial line Depending upon the circuit parameters, the coupling coeirlcient to an external circuit, not shown, may be maintained substantially constant throughout the useful frequency range of the resonator, or the coupling coeificient may be reduced as the frequency is increased.

The device illustrated in Figures 2 and 3 is similar to that described by reference to Figure 1, with the exception that the inner coaxial line conductor [5 includes a telescoping conductor portion 23 formed into an L-shaped loop which is terminated at the adjacent inner wall of the resonator shell 3. Longitudinal adjustment of the tuning piston 5 therefore exposes varying portions of the coupling loop 25 without movement of the conductor comprising the loop.

The device illustrated in Figure 4 is similar to the devices shown in l, 2 and 3 with the exceptions that an insulator 21 supports the coaxial line 9 within the aperture 13 in the piston 5, and the coupling loop is distorted in shape as indicated by the dashed lines 25' as the piston is longitudinally adjusted for tuning the resonator. The end of the coupling loop 25-25' extends into an obliquely positioned depression '28 in the side wall 3 of the resonator.

The device illustrated in Figure 5 provides means whereby longitudinal adjustment of the tuning piston 5 within the resonator shell 3 provides rotation of a coupling loop 3| terminating the coaxial transmission line 9 within the resonator cavity. The coupling loop 3| preferably is of U shape and is terminated at the point 33 on the inner wall of the resonator. The lower portion 35 of the coupling loop is secured to a twisted low-loss semi-rigid insulating strip 31, of polystyrene, mica, or similar materials. The twisted insulated strip 3? extends through the aperture l3 in the piston 5. Longitudinal adjustment of the piston simultaneously varies the frequency of the cavity resonator l and twists the coupling loop 31 to provide adjustment of the coupling coefficient between the coaxial line 9 and the cavity resonator I.

Figure 6 illustrates apparatus wherein adjustment of the frequency of a cavity resonator is provided simultaneously with adjustment in the opposite sense of the coupling coefilcient between said cavity resonator and a waveguide system coupled thereto. The resonator cav1ty opens into a waveguide 39 through a fixed longitudinal aperture 4|. As the piston 5 is advanced toward the end wall ll of the cavity resonator, the side face of the piston gradually reduces the effective length of the coupling aperture 4! thereby correspondingly reducing the coupling coerhcient between the waveguide and the resonator cavity as the resonator frequency is increased.

Figure 7 shows a modification of the embodiment of the invention described by reference to Figure 4 wherein the end of the looped inner conductor is fastened to the inner surface of the res onator whereby the conformation of the loop is varied as the tuning piston position is changed.

The device shown in Figure 7 is preferable to that shown in Figure 4 since a permanent contact is provided at the end of the coupling loop, thereby minimizing erratic contact eiiects which are usually quite objectionable at microwave frequencies.

Figure 8 shows a modification of the devices described by reference to Figures 1 to 5 wherein the shape of the coupling loop is varied as the piston position is changed. The coupling loop 25 is attached to an insulating member it projecting from the end of the resonator chamber whereby the loop is flattened, and hence the coupling is varied, as the piston is advanced. This arrangement permits various non-linear coupling characteristics, depending upon the original loop conformation.

Figure 9 shows a modification of the embodiment of the invention described heretofore by reference to Figure 6, wherein the aperture shape is other than rectangular. For example, the triangular aperture ll' illustrated in Figure 9 provides a non-linear coupling characteristic. Other aperture shapes may be provided in accordance with known microwave technique.

Figure 10 is a further modification of the embodiment of the invention described by reference to Figure 6 wherein the aperture extends along the magnetic axis of the waveguide 33', adjustment of the piston providing control of the length of said magnetic axis instead of variation of the length of the electrical axis as in the device of Figure 6.

It should be understood that the variableor constant-coupling systems disclosed herein may be employed as either output or input coupling elements of associated circuits and that simultaneous or separate control of a plurality of such coupling elements may be provided for a single cavity resonator or for a plurality of cavity resonators, By proper proportioning of the adjustable resonator and the associated coupling probe, loop or aperture device, the coupling coefficient to an external circuit may be maintained substantially constant or varied in opposite sense to the adjustment of the resonator frequency.

Thus the invention disclosed comprises several embodiments of cavity resonator, wide-frequency-band, microwave devices providing tuning by adjustment of a tuning piston wherein such piston adjustment provides corresponding control of the coupling coefiicient between said resonator and an external circuit.

I claim as my invention:

1. A variable-coupling wide-frequency-band microwave resonator including a hollow resonator, means including a movable piston having an aperture therein disposed within said resonator for tuning said resonator, and a coupling element extending into said resonator for microwave coupling to said resonator, said coupling element including a coupling loop and an integral twisted insulating strip, said strip extending into said resonator through said aperture in said piston, whereby adjustment of said tuning piston simultaneously varies in opposite sense the resonant frequency of said resonator and the coupling coeflicient of said element, said strip and said 100p being rotated as said piston is adjusted longitudinally.

2. A variable-coupling wide-frequency-band 20 microwave resonator including a hollow resonator, means including a movable piston having an aperture therein disposed within said resonator for tuning said resonator, a coupling loop extend- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,253,503 Bowen Aug. 26, 1941 2,281,550 Barrow May 5, 1942 2,311,520 Clifford Feb. 16, 1943 2,401,634 Gubin June 4, 1946 2,411,424 Gurewitsch Nov. 19, 1946 2,417,542 Carter Mar. 18, 1947 2,426,177 Carlson Aug. 26, 1947 2,427,107 Landon Sept. 9, 1947 

